1. Field of the Invention
This invention relates generally to electrically small antenna, and more particularly to electrically small antenna systems suitable for waves propagating through water.
2. Description of the Related Art
Electrically large antennas with dimensions of a fraction of a wavelength are well known for efficient launch and reception of electromagnetic signals. The half wave dipole and quarter wave antenna are typical examples. However, in some instances, particularly in low frequency and portable applications, the dimensions of a wavelength related antenna become impractical.
Underwater radio applications differ from through air communications primarily due to the conductive nature of the medium. Seawater has a higher ionic salt content than fresh water hence much higher conductivity. In the radio spectrum, attenuation of electromagnetic signals increases rapidly with frequency. Relatively low signal frequencies (1 Hz to 10 MHz) are commonly employed in communication systems underwater in order to reduce signal attenuation as much as possible.
The wavelength of a propagating wave is greatly shortened in a partially conductive medium compared to air or free space. For example, at 1 kHz the free space wavelength is 300 km while in seawater this is reduced to 50 m. However, these are still impracticably large dimensions for the wavelength related antennas conventionally used in through-air radio systems. In the past, portable underwater radio systems have used loop antennas that are small compared to their operating wavelength. These loops approximate to a magnetic dipole.
A magnetic loop carrying an alternating current produces three distinct field components. In addition to conductive attenuation, each term has a different geometric loss as we move distance r from the launching loop. An inductive term varies with a coefficient that includes a 1/r3 term, a quasi-static term by 1/r2 and a propagating wave by 1/r. In designing a communicating system it is preferable to maximise the propagating 1/r component since this dissipates at the lowest rate of the three terms. Equivalent distance varying terms exist for an electric field source.
In the magnetic dipole every element of the loop has a corresponding element at the other side of the loop with current flowing in the opposite direction. In the limit of a loop radius, which is a fraction of a wavelength, propagating waves emitted by these elements are in opposing phase, so exactly cancel and no 1/r propagating term is launched. Electrically small loop antennas are therefore intrinsically poor generators or receivers of propagating radio waves.
There is a general need for an electrically small antenna which optimises launch of the propagating field component to improve antenna efficiency and achievable range. This is of particular interest for underwater radio applications.